The present invention relates to an improved method for the production of alumina hydrate from bauxite according to the Bayer method wherein the properties of the coarse alumina hydrate crystals as well as the yield, with respect to time, are improved. Also, the quantity of seed crystals required for the precipitation of a defined quantity of alumina hydrate can be reduced.
It is known that in the European Bayer process, ground bauxite is heated in an autoclave together with a circulating liquor containing sodium hydroxide and sodium aluminate, whereby alumina goes into solution and red mud remains in undissolved form. The decomposition liquor leaving the autoclave is diluted with wash liquor and/or stirred-out sodium aluminate liquor and the red mud is separated from the liquor. After cooling and inoculation with aluminum hydrate crystals, the alumina hydrate is separated from the separated liquor at temperatures from 50.degree. to 70.degree. C.
A significant feature of this European process is that a concentrated sodium hydroxide solution, having a sodium hydroxide concentration of 280 to 450 g/l, calculated as Na.sub.2 CO.sub.3, is used for the decomposition and, after dilution, the resulting aluminum oxide hydrate is stirred out at lower temperatures. An aluminum oxide hydrate stirred out under these conditions appears in finely crystalline form. Part of this product is calcined to alumina and another part is used for the inoculation. Due to considerable dust losses during calcination, transport and electrolysis, such a product is undesirable today. Moreover, the fine grained aluminum oxide only poorly absorbs the fluorine gas developed during electrolysis while coarse grained aluminum oxide is well able to do so. The result of the latter is that fluorine gas losses are reduced and considerably less fluorine gas is transferred to the atmosphere which facilitates compliance with increasingly stricter environmental protection regulations. Additionally, in various fields, e.g. utilization as grinding agent and catalyst, the use of coarsely crystalline aluminum oxide is preferred in many cases.
In contrast thereto is the American Bayer process in which sodium hydroxide in the decomposition liquor is concentrated at about 160 to 225 g/l, calculated as Na.sub.2 CO.sub.3, which is considerably below the concentration in the European decomposition liquors. In this process, however, aluminum oxide hydrates result which have different grain sizes, with the coarse crystals being extracted as product and calcined while the fine and medium coarse crystals are used as seed crystals.
Due to the different types of bauxite decomposed in the above-described variations of the Bayer process, the American Bayer process cannot easily be transferred to bauxites processed in Europe.
Moreover, the American process operates with diluted liquors to produce aluminum oxide hydrates with different grain sizes, which has the drawback that liquor productivity is poorer than if a concentrated liquor were used. With a diluted liquor, more liquid must be processed and turned over than with a concentrated liquor which results in a correspondingly larger requirement for apparatus and higher manufacturing costs and which also requires more energy.
There thus arises the problem of eliminating the above-described drawbacks of the Bayer process and finding a process which permits the production of coarse grained aluminum oxide hydrate, also in more concentrated mother liquors.
To solve this problem, Canadian Pat. No. 1,098,284 proposes to produce product crystals and seed crystals under different conditions in different stirrers. Thus, in a product cascade, essentially coarsely crystalline alumina hydrate is to be produced with continuous parallel supply of fresh (not yet stirred out) supersaturated mother liquor into the individual stirrers in the presence of medium coarse seed crystals, and in an inoculation cascade, finely crystalline alumina hydrate is to be produced from the parallel extracted overflows of the product cascade. The inoculation material and the aluminum oxide hydrate formed while passing through the product cascade are conducted in series through the stirrers of the cascade. The significant feature of this process is that in the product cascade, high solids concentrations (solids-aluminum hydroxide) are present in highly supersaturated sodium aluminate liquors and the process takes place at temperatures from 75.degree. to 90.degree. C. Under these conditions, crystal enlargement is predominant due to crystal growth, while nucleation and seed crystal formation are repressed. In contradistinction thereto, the inoculation cascade operates at temperatures from 50.degree. to 65.degree. C. and the formation of finely crystalline aluminum oxide hydrate in partially stirred-out sodium aluminate liquors is enhanced.
In the prior art Canadian process, the supersaturated mother liquor yields coarsely crystalline aluminum oxide hydrate in amounts from 72 to 85 g/l, expressed as Al.sub.2 O.sub.3 per m.sup.3 of liquor circulated. The product contains no more than 1% crystals of a size smaller than 45 microns.
The sodium hydroxide contained in the supersaturated sodium aluminate liquor of the prior art Canadian process is present in quantities from 200 to 300 g/l, particularly 240 g/l, calculated as Na.sub.2 CO.sub.3.
A further feature of the process according to Canadian Pat. No. 1,098,284 is that the product cascade operates with three-phase stirrers. In such stirrers, the lowermost phase I constitutes a settling zone with high solids content (40 to 65%), the middle zone II constitutes the stir-out zone with a solids content of 10 to 20%, and the upper zone III, which has a solids content of 1 to 5%, is a clarification zone. Although separation into phases, which can be realized without difficulty with the aid of the stirring device disclosed in that patent, leads to the formation of coarsely crystalline aluminum oxide hydrate in concentrated sodium aluminate liquors (the NaOH content is higher than in the American process and is at approximately the same level as in the European Bayer process), it has been found that the improved liquor productivity is not sufficient to solve the problem at hand. Stirring times of at least 50 hours are often considered too long in practice. In particular, it has been observed that the coarse hydrate crystals produced according to the cited Canadian prior art process are less stable and break easily, in many cases already in the fluidized bed of the calcining furnace.